An airbag is a vehicle safety device that generally includes a flexible envelope (e.g., a nylon fabric) designed to inflate rapidly during an automobile collision. The airbag's purpose is to cushion occupants during a crash and provide protection to their bodies when they strike interior objects such as the steering wheel or a window. Modern vehicles may contain multiple airbags in various side and/or frontal locations of the passenger seating positions, and sensors may deploy one or more airbags in an impact zone at variable rates based on the type and the severity of impact.
Most airbag designs are inflated by the ignition of a gas generator propellant via a pyrotechnic device to rapidly inflate the flexible envelope. The pyrotechnic device can generally include an electrical conductor wrapped in a combustible material and can activate quickly (e.g., less than 2 milliseconds) with a current pulse (e.g., of about 1 to about 3 amperes). When the conductor becomes hot enough, it ignites the combustible material (e.g., a solid propellant), which initiates the gas airbag (e.g., typical rate of inflation in current technology is about 20 to about 30 milliseconds). For example, the pyrotechnic device can include a propellant mixture of sodium azide (NaN3), potassium nitrate (KNO3), and silica dioxide (SiO2), which can react in three separate reactions to produce nitrogen gas. The reactions, in order, are as follows:2NaN3→2Na+3N2 (g);  (1)10Na+2KNO3→K2O+5Na2O+N2 (g);  (2)K2O+Na2O+2SiO2→K2O3Si+Na2O3Si (silicate glass).  (3)
The first reaction is the decomposition of NaN3 under high temperature conditions using an electric impulse. This impulse generates in excess of 300° C. temperatures required for the decomposition of the NaN3 which produces Na metal and N2 gas. Since Na metal is highly reactive, the KNO3 and SiO2 react and remove it, in turn producing more N2 gas. The second reaction shows the additional generation of N2 gas. The reason that KNO3 is used, rather than something like NaNO3, is because it is less hygroscopic. Absorbed moisture can de-sensitize the system and cause the reaction to fail; therefore, it is important that the materials used in this reaction are not hygroscopic. The final reaction eliminates the highly reactive metal oxides, K2O and Na2O, produced in the second reaction. These metal oxides react with SiO2 to produce a silicate glass, a harmless and stable compound. Other similar pyrotechnic device include combustible material as the propellant that produce gases and particles having extreme temperatures (e.g., about 500° C.).
As a result of these (or other, similar ignition reactions), the inner area of the airbag is exposed to hot gas as well as hot particulates formed during the ignition processes. These hot gases and hot particulates are particularly concentrated in the area within the airbag proximate to the pyrotechnic device. As a solution, attempts have been made to include a sacrificial fabric within the airbag in the area proximate to the pyrotechnic device to help protect the airbag fabric. In use, this sacrificial fabric is burnt by the hot gas and/or hot particulates. However, due to the need to ensure that the sacrificial layer can sufficiently protect the airbag fabric, multiple layers of the sacrificial layer is included (either unbonded or as a heat shield) within the device depending upon the combustion temperature expected at inflation. Thus, these sacrificial heat shield fabrics add significant thickness to the construction of the airbag device. Additionally, such sacrificial heat shield fabrics add significant cost to the airbag device.
As such, a need exists for an improved heat shield material to protect the airbag material from burning upon ignition of the pyrotechnic device.